Testing for Chlorine and Bromine

May 2024

There appears to be some confusion arising from the following paragraph found in the revised HSG274 part 1, Info Box 1.5: Biocide types and application:

When using the DPD No 1 methodology it is essential that the result is pH-adjusted to account for the conversion of hypochlorous and hypobromous acids into their less effective hypochlorite and hypobromite ions, when higher pH levels are present.

This is important, as the DPD No 1 test is not specific to either and will pick up some other oxidants that may be present in the system.

This statement does not require the pH of the sample to be adjusted before testing as this will not change the answer but will require some additional adjustments prior to interpreting the data as detailed in HSG274 part 1. The above statement in HSG274 Part 1 refers to the need to measure the pH of the water system (the sample) and use the graphs and table below to determine how much halogen is present in its more biocidal/effective acid form.

The critical control limits required to achieve a desired outcome by LCA Members must be based on the effective dose of biocide and the desired microbial control. This should be based on the effective dose of hypochlorous acid (HOCl) or hypobromous acid (HOBr). All control limits should be included in the written control scheme for the system and recorded as a control limit on all relevant reports. It must be clear from the records of testing, what is being tested, what the control limit being applied is, and whether the result is within control limits.